Generally, a magnetic-type position detector detects a change in the position of a detection object relative to a reference part. The magnetic-type position detector may utilize a magnetic flux generator such as a magnet. For example, a position detector disclosed in a patent document 1 (i.e., Japanese Patent Laid-Open No. JP-A-H10-104044) is configured to form a closed magnetic circuit having two magnets and two magnetic flux transmission parts that are disposed on a reference part. In such structure, the two magnets are respectively bound by the ends of the two mutually-facing magnetic flux transmission parts. A flow of spill magnetic fluxes from one transmission part to the other occurs within a gap between the respective ends of the two magnetic flux transmission parts. A magnetic flux density detector is configured to move together with the detection object within the gap between the two magnetic flux transmission parts, and to output a detection signal according to the density of the magnetic flux passing therethrough. In such manner, the position detector detects the position of the detection object relative to the reference part based on an output signal that is output from the magnetic flux detector.
In the position detector of the patent document 1, the magnetic flux flows from one magnetic pole to the other in a flying manner around the magnets. Therefore, the absolute value of the density of the magnetic flux which passes through the magnetic flux density detector steeply increases at an end portion of the gap between the two magnetic flux transmission parts (i.e., within a proximity of the magnets). Such magnetic flux may deteriorate the linearity of the signal outputted from the magnetic flux density detector, and may lower the position detecting accuracy at the end portion of the movable range of the detection object.
Therefore, the position detector of the patent document 1 uses bypass yokes that are disposed in contact with both end portions of the magnetic flux transmission parts and the magnets, for forcefully flowing and guiding the “flying” magnetic flux which are otherwise flying from one magnetic pole to the other. In such manner, the steep rise of the absolute value of the magnetic flux density at the end portion of the gap between the two magnetic flux transmission parts is prevented, and the linearity of the signal outputted from the magnetic flux density detector is secured for a wide movable range of the detection object.
However, the position detector of patent document 1, in which the bypass yokes in contact with the end portions of the two magnetic flux transmission parts and the magnets, the magnetic flux generated from the magnet may largely flow to the bypass yoke, thereby causing a decrease of the magnetic flux flowing in the two magnetic flux transmission parts. When the amount of the magnetic flux flowing in the magnetic flux transmission parts decreases, the amount of the spill magnetic flux flowing in the gap between the magnetic flux transmission parts also decreases. In such case, the dynamic range of the magnetic flux density detected by the magnetic flux density detector decreases, which may lead to the deteriorated position detection accuracy of the position detector having such structure.